Multiple challenges face drug development today including high costs and long development cycles for new therapeutics. Methods that promote accelerated drug development are urgently needed. The efficacy of therapeutics is dependent on a drug binding to its target. Due to its simplicity, a protein thermal shift assay (TSA) is a commonly used method for screening libraries and validating hits in drug discovery programs. The most common method of TSA in use today can only be performed using purified protein, which has several disadvantages. Recently, the cellular thermal shift assay (CETSA) was developed to detect endogenous target protein within cells or cell lysates, thereby alleviating the need to create purified proteins and allowing for target engagement analysis in complex environments that are more biologically relevant. However, the CETSA is a multi-step assay in which the analysis of target engagement relies upon western blot or AlphaScreen (Perkin Elmer) technologies, both of which have several disadvantages, e.g., cost, insensitive, multi-stepped protocols, requires cell lysis and spin steps, low throughput (e.g., Western blot), dependent upon antibody recognition of native protein state, cell line optimization, requires a large amount of cells, etc. What is needed is a simple, homogeneous, rapid, and inexpensive TSA, whereby target engagement can be characterized in complex environments, such as cells and cell lysate.